This invention relates to an improved solid golf ball.
On solid golf balls having solid cores, a number of attempts have been made to soften the feel of the ball when hit. One common approach is to soften the core. At present, this approach requires use of a relatively hard cover in order to compensate for a loss of resilience. Undesirably, the hard cover tends to reduce the substantial dependency of resilience on speed that the soft core possesses.
In general, the substantial dependency of resilience on speed of golf balls means that the resilience at high head speeds is low, but a very good resilience is exerted in a low head speed region. Such balls are suited for low head speed players.
The soft core technology known thus far, however, fails to impart to the ball high resilience at low head speeds because of the influence of the hard cover.
An object of the invention is to provide a solid golf ball having improved flight performance in a low head speed region and a soft feel when hit.
The invention is directed to a solid golf ball comprising a solid core and a cover enclosing the core. It was empirically found that if the firing velocity used in the measurement of a coefficient of restitution (COR) and the head speed (HS) upon hitting with a given club are of the same value, the deformation behaviors of the ball upon impact are substantially identical between the measurement of COR and the club hitting. It has been found that by specifying the COR at a firing velocity of the golf ball and the difference of COR in a low HS region (typically 25 m/s) and a high HS region (typically 50 m/s) of both the ball and a ball structure prior to formation of the cover, quite unexpectedly there is obtained a golf ball which is improved in flight performance in the low head speed region without sacrificing the flight performance in the high head speed region and at the same time, has a soft feel when hit.
It has also been found that by specifying the Shore D hardness of the cover material relative to the difference in COR of the ball structure prior to formation of the cover between the low and high HS regions, the gage of the cover, and the contact area of the ball when hit at a velocity of 50 m/s, the speed dependency of resilience of the ball when hit at a relatively low head speed can be enhanced and the above-mentioned advantages are more effectively achievable without sacrificing the resilience or rebound in the high head speed region.
Accordingly, the invention provides a solid golf ball comprising a solid core and a cover. Provided that a coefficient of restitution at a firing velocity of v m/s is designated by CORv, a ball structure prior to formation of the cover satisfies COR25xe2x88x92COR50xe2x89xa70.100, and the ball satisfies COR50xe2x89xa70.740 and COR25xe2x88x92COR50xe2x89xa70.09.
In one preferred embodiment, the cover is formed to a gage of 1.2 to 3.0 mm and of such a material that the Shore D hardness of the cover is not greater than 327.61X+27.239 wherein X represents (COR25xe2x88x92COR50) of the ball structure prior to formation of the cover. Preferably, the ball structure prior to formation of the cover undergoes a deflection of 2.8 to 5.0 mm under an applied load of 100 kg. Also preferably, the solid golf ball has a contact area of 5.0 to 6.5 cm2 when fired at a firing velocity of 50 m/s.
The solid golf ball of the invention has at least a spherical solid core and a cover enclosing the core in a concentric fashion. The respective components may be formed of well-known materials by conventional methods.
The core may be formed of well-known rubber compositions. For example, a rubber composition comprising polybutadiene as the base is preferred. The polybutadiene used herein is preferably cis-1,4-polybutadiene having a cis structure of at least 40%. Where desired, another suitable rubber component such as natural rubber, polyisoprene rubber or styrene-butadiene rubber may be compounded with the polybutadiene to give the base rubber. Increasing the polybutadiene component leads to an improvement in resilience. Preferably less than about 10 parts by weight of the other rubber component is blended per 100 parts by weight of polybutadiene.
A crosslinking agent may be included in the rubber composition. Exemplary crosslinking agents are the zinc and magnesium salts of unsaturated fatty acids, such as zinc dimethacrylate and zinc diacrylate, and ester compounds such as trimethylpropane methacrylate. Zinc diacrylate is especially preferred for achieving a high resilience. The crosslinking agent is preferably included in an amount of about 10 to about 40 parts by weight per 100 parts by weight of the base rubber.
A vulcanizing agent is generally compounded in the rubber composition. Peroxides are preferred vulcanizing agents. It is recommended that the vulcanizing agent include a peroxide having a one minute half-life temperature of not higher than 155xc2x0 C., and such peroxide account for at least about 30%, more preferably about 40 to 70% by weight of the entire vulcanizing agent. Examples of suitable peroxides include commercially available products such as Perhexa 3M (dicumyl peroxide, manufactured by Nippon Oils and Fats Co., Ltd.). The amount of vulcanizing agent included in the rubber composition is preferably from about 0.6 to about 2 parts by weight per 100 parts by weight of the base rubber.
If necessary, other suitable ingredients may also be incorporated in the rubber composition, such as antioxidants and fillers (e.g., zinc oxide, barium sulfate) for modifying the specific gravity. The amount of the gravity adjuster blended is typically about 1 to 30 parts by weight per 100 parts by weight of the base rubber.
Production of the core from the rubber composition may be carried out by a known method involving molding and vulcanizing or curing steps.
Around the core, an intermediate layer can be formed from a similar rubber composition or a thermoplastic resin base composition. Specifically, the intermediate layer may be formed by using a suitable material selected from among ionomer resins and thermoplastic elastomers such as polyurethane elastomers, polyamide elastomers and polyester elastomers, and molding the material around the core by a conventional injection molding process. The intermediate layer may be formed to a single layer or a multilayer structure of two or more layers.
As used herein, the term xe2x80x9cball structure prior to formation of the coverxe2x80x9d means the core itself when the golf ball has a two-layer structure consisting of a core and a cover. When the golf ball further has an intermediate layer between the core and the cover as mentioned just above, a sphere having the core enclosed with the intermediate layer is meant by the ball structure.
The ball structure prior to formation of the cover preferably has a deflection of 2.8 to 5.0 mm, more preferably 3.0 to 4.8 mm, and most preferably 3.0 to 4.6 mm, under an applied load of 100 kg. A deflection of less than 2.8 mm may lead to a too high hardness and make it difficult to increase the speed dependency of resilience. A deflection in excess of 5.0 mm may lead to poor resilience.
Though not critical, the ball structure prior to formation of the cover generally has an outer diameter of 36.7 to 40.3 mm, and especially 37.5 to 39.8 mm.
The ball structure prior to formation of the cover may be a single layer (that is, the core alone) or have a multilayer structure of two or more layers (that is, the core enclosed with the intermediate layer or layers), with the multilayer structure being preferred. In the multilayer embodiment, the ball structure as a whole may fall within the above-indicated ranges of the deflection under 100 kg load and the outer diameter. Where the intermediate layer is formed, it preferably has a thickness or gage of 0.5 to 5.0 mm, and especially 1.0 to 4.5 mm.
According to the invention, the ball structure prior to formation of the cover and the ball each should have a specific difference between the coefficients of restitution (COR) at predetermined firing velocities.
Coefficient of restitution (COR) is measured by firing a golf ball or a ball structure prior to formation of the cover in a pneumatic cannon at a velocity (referred to as firing velocity) against a steel plate which is positioned apart from the muzzle of the cannon. The rebound velocity is then measured. The rebound velocity is divided by the forward velocity to give the coefficient of restitution. A COR value which is more approximate to unity (1) indicates higher resilience. The invention uses the nomenclature that a coefficient of restitution at a firing velocity of v m/s is designated by CORv. That is, COR""s at firing velocities of 25 m/s and 50 m/s are designated COR25 and COR50, respectively.
According to the invention, the ball structure prior to formation of the cover should satisfy COR25xe2x88x92COR50xe2x89xa70.100, preferably COR25xe2x88x92COR50xe2x89xa70.105, and more preferably COR25xe2x88x92COR50xe2x89xa70.110. If this difference is less than 0.100, the speed dependency of resilience of the ball cannot be enhanced.
Specifically stated, it is recommended that the ball structure prior to formation of the cover have a COR value of 0.80 to 0.90, especially 0.82 to 0.88 at a firing velocity of 25 m/s, and 0.70 to 0.80, especially 0.72 to 0.78 at a firing velocity of 50 m/s. Outside the ranges, it may be difficult to provide the desired difference between COR25 and COR50.
The golf ball of the invention is obtained by forming the cover around the ball structure prior to formation of the cover (i.e., the core alone or the core enclosed with the intermediate layer). According to the invention, the ball should satisfy COR25xe2x88x92COR50xe2x89xa70.090, preferably COR25xe2x88x92COR50xe2x89xa70.095, and more preferably COR25xe2x88x92COR50xe2x89xa70.100. If this difference is less than 0.090, the speed dependency of resilience of the ball cannot be enhanced.
Additionally, the invention requires that the ball have a COR of at least 0.740 at a firing velocity of 50 m/s (that is, COR50xe2x89xa70.740), preferably at least 0.745 and more preferably at least 0.750 at a firing velocity of 50 m/s. The COR of the ball at a firing velocity of 25 m/s need not be specifically limited although the ball preferably have a COR25 of at least 0.830, more preferably at least 0.835, and most preferably at least 0.840. Outside the ranges, it may be difficult to provide the optimum difference between COR25 and COR50 and the desired speed dependency of resilience of the ball.
The golf ball of the invention is prepared by enclosing the core or the core plus the intermediate layer with the cover. The cover may be made of well-known cover stocks such as thermoplastic resins. Suitable materials include ionomer resins and thermoplastic elastomers such as polyurethane elastomers, polyamide elastomers and polyester elastomers.
In forming the cover, a choice is preferably made of a material having an appropriate hardness for the predetermined difference of coefficient of restitution of the ball structure prior to formation of the cover. Provided that X represents (COR25xe2x88x92COR50) of the ball structure prior to formation of the cover and D represents the Shore D hardness of the cover material, it is recommended that the cover material satisfy
Dxe2x89xa7327.61X+27.239.
Specifically, the cover material should preferably have a Shore D hardness of 50 to 65, and especially 53 to 62. A hardness outside the range may detract from the feel and fail to provide good spin performance enough for skilled golfers to accept.
The thickness or gage of the cover is not critical although the cover gage is generally 1.2 to 3.0 mm, preferably 1.5 to 2.5 mm, and especially 1.6 to 2.1 mm. A gage of less than 1.2 mm may detract from ball durability. A cover gage of more than 3.0 mm may offset the speed dependency of resilience of the core, failing to attain the objects of the invention.
Most often, the cover is formed by injection molding of the above-described materials. Of course, other well-known molding techniques such as compression molding are employable.
In the practice of the invention, the golf ball satisfying the above-mentioned requirements of COR and COR difference can be obtained by properly selecting the material type, vulcanizing conditions and deflection of the core, the material type, hardness and gage of the optional intermediate layer, and the material type, hardness and gage of the cover.
The golf ball of the invention can be prepared by injection molding the above-described cover material around the ball structure prior to formation of the cover in a conventional manner. Like conventional golf balls, the golf ball of the invention has numerous dimples formed in the surface of the cover. The shape and arrangement of dimples are selected as appropriate from well-known ones.
The golf ball of the above construction preferably has a deflection of 2.4 to 3.8 mm and especially 2.6 to 3.5 mm under an applied load of 100 kg.
Further preferably, the golf ball has a contact area of 5.0 to 6.5 cm2, more preferably 5.2 to 6.3 cm2, and most preferably 5.4 to 6.0 cm2, when fired at a firing velocity of 50 m/s. A contact area of less than 5.0 cm2 may lead to a hard feel whereas a contact area of more than 6.5 cm2 may detract from resilience.
The inventive golf ball may be formed so as to have a diameter and weight which conform with the Rules of Golf, that is, a diameter of not less than 42.67 mm and a weight of not greater than 45.93 g.